Descr.tuple_fetch returns :badindex for valid access on difference types

[lukaszsamson]

Existing issue

• I have searched existing issues and could not find a duplicate.

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.4.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Interactive Elixir (1.21.0-dev)

Operating system

any

Current behavior

The following code shows the bug:

# create a set of all tupels of size >= 1
t = opt_difference(open_tuple([]), tuple([]))
%{
  tuple: {3186801813, {-113427502, :closed, []}, :bdd_bot, :bdd_bot,
   {61102469, :open, []}}
}

# which is equal to open tuple of size 1
equal?(t, open_tuple([term()]))
true

# the representation is different though
open_tuple([term()])
%{tuple: {18315736, :open, [:term]}}

# and fetch is not working correctly on the difference type

tuple_fetch(t, 0)
:badindex

tuple_fetch(open_tuple([term()]), 0)
{false, :term}

Likely reason:

elixir/lib/elixir/lib/module/types/descr.ex

Lines 4982 to 4992 in fd1109e

	value =
	if tuple_pair_projection_keeps_full_fst?(negative, bdd) do
	value
	else
	negs
	|> tuple_split_negative(index, value, bdd)
	|> Enum.reduce(none(), fn {value, _}, acc -> opt_union(value, acc) end)
	end
	{optional?, descr} = pop_optional_static(value)
	{optional? or acc_optional?, opt_union(descr, acc_descr)}

shortcut value returned from tuple_pair_projection_keeps_full_fst? is taken unconditionally and carries optional?. Negations that remove small arity tuples can never cancel it

Expected behavior

tuple_fetch should work correctly on difference type